Anak Krakatoa, which was 338 metres high, is now only 110 metres tall after triggering tsunami that killed over 400.
Indonesia’s most volatile volcano on Sunday spewed ash and hot gas in a massive column as high as 6km (3.7 miles) into the sky.
Mount Merapi’s clouds of ash, accompanied by a rumbling sound heard kilometres (miles) away, blanketed several villages on the main island of Java.
Indonesia’s Center for Volcanology and Geological Hazard Mitigation did not raise Merapi’s alert status, which already was at the third-highest level since it began erupting last August.
In March, an eruption triggered the closure of the airport in Solo on the densely populated Java, authorities said. Since the March eruption, the local disaster mitigation agency has been warning people to keep out of a 3km (1.8 miles) exclusion zone around Merapi.
The 2,968-metre (9,737-foot) mountain is the most active of 500 Indonesian volcanoes. It has rumbled and generated dark hot clouds since last year.
Its last major eruption in 2010 killed 353 people.
Indonesia, an archipelago of 270 million people, is prone to earthquakes and volcanic activity because it sits along the Pacific Ring of Fire, a horseshoe-shaped series of fault lines around the ocean.